Use macos-14 for CPython tests.

Author: freakboy3742

.github/workflows/ci.yaml

@@ -136,7 +136,7 @@ jobs:
             - briefcase-run-args:
 
             - platform: iOS
-              briefcase-run-args: ' -d "iPhone 16e"::iOS 18.5"'
+              briefcase-run-args: ' -d "iPhone 16e::iOS 18.5"'
 
     steps:
     - uses: actions/checkout@v5
@@ -184,7 +184,8 @@ jobs:
 
   cpython-testbed:
     name: CPython testbed (${{ matrix.platform }})
-    runs-on: macOS-15
+    # For now, CPython testbed can't run on macos-15: https://github.com/actions/runner-images/issues/12777
+    runs-on: macOS-14
     needs: [ config, build ]
     strategy:
       fail-fast: false
@@ -197,15 +198,6 @@ jobs:
     steps:
     - uses: actions/checkout@v5
 
-    - name: Set up Xcode
-      # GitHub recommends explicitly selecting the desired Xcode version:
-      # https://github.com/actions/runner-images/issues/12541#issuecomment-3083850140
-      # This became a necessity as a result of
-      # https://github.com/actions/runner-images/issues/12541 and
-      # https://github.com/actions/runner-images/issues/12751.
-      run: |
-        sudo xcode-select --switch /Applications/Xcode_16.4.app
-
     - name: Get build artifact
       uses: actions/[email protected]
       with:
@@ -239,7 +231,7 @@ jobs:
         # - test_os as a test of system library calls
         # - test_bz2 as a simple test of third party libraries
         # - test_ctypes as a test of FFI
-        python -m testbed run ${{ matrix.testbed-args }} -- test --single-process --rerun -W test_builtin test_grammar test_os test_bz2 test_ctypes
+        python -m testbed run -- test --single-process --rerun -W test_builtin test_grammar test_os test_bz2 test_ctypes
 
   crossenv-test:
     name: Cross-platform env test (${{ matrix.multiarch }})